In recent years, electronic appliances have become strikingly small and light. In connection therewith, the battery used therein as an electric source is strongly required to be small and light as well. Various secondary batteries have been developed so as to satisfy such a requirement; and nickel-hydrogen secondary battery and lithium ion secondary battery, for example, have been put to practical use.
As the process for producing an electrode which is a constituent member of the above secondary batteries, there is a process which comprises kneading an active material such as hydrogen absorbing alloy or graphite, carboxymethyl cellulose as a thickening agent, a styrene-butadiene copolymer latex as a binder and water as a dispersing medium to prepare a paste and coating the paste on the surface of a collector, followed by drying (for example, refer to Patent Documents 1 and 2).
In the above, the binder functions so as to increase the adhesivity between the electrode layer containing the active material and the collector. The styrene-butadiene copolymer latex, however, has not been fully satisfactory in the adhesivity with the collector. With a secondary battery using an electrode in which the adhesivity between electrode layer and collector is not sufficient, there is a problem that it is impossible to enhance the battery properties including charge-discharge cycle property.
As a prior art relevant to the problem, there is disclosed use, as a binder for lithium secondary battery, of a modified polymer obtained by introducing a functional group such as carboxyl group into a hydrogenated, diene polymer (for example, refer to Patent Document 3). Even with the binder disclosed in the Patent Document 3, however, the adhesivity between electrode layer and collector is not enhanced sufficiently. Further, in a secondary battery using an electrode of insufficient adhesivity, a reduction in capacity particularly in rapid discharge is striking.
Meanwhile, for alleviation of capacity reduction in rapid discharge (cycle property), there is disclosed an aqueous dispersion of a composite polymer obtained from a fluorine-containing polymer and an acrylic polymer containing a functional group such as carboxyl group (for example, refer to Patent Document 4). However, the electrode paste prepared using the aqueous dispersion of composite polymer, disclosed in the Patent Document 4, tends to generate a precipitate in some cases, when allowed to stand for a long period; and its stability of paste need be improved.    Patent Document 1: JP-A-1999-7948    Patent Document 2: JP-A-2001-210318    Patent Document 3: JP-A-1998-17714    Patent Document 4: JP No. 3601250